(a) Field of the Invention
This invention relates to a digital color encoder for producing such composite color video signals as an NTSC signal directly from digitized primary color singals.
(b) Description of the Prior Art
Generally a color picture signal E(t) of an NTSC system is given by the following formula EQU E(t)=Y+0.493 (B-Y) sin .omega.t+0.877 (R-Y) cos .omega.t (1)
wherein Y represents a luminance signal, B-Y represents a color difference signal of blue and R-Y represents a color difference signal of red. These are given by the following formulae from primary color signals R, G and B of red, green and blue colors respectively: EQU Y=0.299R+0.587G+0.114B (2) EQU B-Y=-0.299R-0.587G+0.886B (3) EQU R-Y=0.701R-0.587G-0.114B (4)
In the formula (1), .omega.=2.pi.f=2.pi. .times.3.579545 MHz, where f represents a chrominance subcarrier frequency (=3.579545 MHz).
In a conventional digital color encoder, digitized primary color signals R, G and B are converted to a luminance signal Y and chrominance signals I and Q corresponding to color difference signals R-Y and B-Y; and then a composite color video signal is formed through modulation of a chrominance subcarrier by means of an analog balanced modulator or the like.
In this system, signal sources are individually required, the freedom in the color picture image is low on the digitizing process and computation process, the circuit formation is complicated and the cost of construction is very high.